Mouse Track

I've put together code from W3 Schools and from Javascript.Info to see how
mouse tracking and free drag on elements works: kinda fun. I might be able
to run a simple puzzle without recourse to JQuery as a result...

https://plnkr.co/edit/LU1jCxgKiYndRc27DEu6?p=info

                                    *     *     *

The solution:

The function needs to run from the body tag, and use pageX, pageY.
That's it.